Circadian rhythms and environmental lighting regulate a number of endocrine and behavioral functions. Cyclic GMP mimics, and may mediate, the effects of light on the circadian oscillator in the eye of Aplysia. Monoclonal antibodies directed against rhodopsin permit visualization of photopigment in chick pineal, and differentiate among photopigment containing cells in chick and rat retina. Monoclonal antibody directed against the G protein regulating phosphodiesterase in retina also recognizes the G protein regulating adenylate cyclase in the rat pineal, indicating similarity (if not identify) of these proteins.